Ion channels are “molecular gates” that regulate the flow of ions into and out of cells. Ion flow plays an important role in all brain cell communication necessary for learning and memory. Additionally, ion flow is important in many physiological processes including, but not limited to, heart rate and body movement. Aberrations in ion channels have been implicated in, amongst other disorders, epilepsy, schizophrenia, Alzheimer's disease, migraine, arrhythmia, diabetes, and stroke damage. Ions flow down their electrochemical gradient through the ion channels (passive transport). The core of the channel is hydrophilic, and contains a part of the protein, the selectivity filter, which recognizes only certain ions and allows them to pass through. Channels are named by the ion(s) they allow to pass. Examples of ion channels include, but are not limited to, calcium channels, potassium channels, sodium channels, chloride channels, etc. An additional component of the channel is the gate. Only when the gate is open can the ions recognized by the selectivity filter pass through the channel. Gates open in response to a variety of stimuli, including, but not limited to, changes in membrane potential or the presence of certain chemicals outside or inside the cell. Channel names often also include an indication of what controls the gate: e.g., “voltage-gated calcium channel.” Presently, more than 50 different types of ion channels have been identified.
Communication between neurons is achieved by the release of neurotransmitters into the synapse. These neurotransmitters then activate receptors on the post-synaptic neuron. Many such receptors contain pores to rapidly conduct ions, such as sodium, calcium, potassium, and chloride, into the neuron. These pores, or channels, are made of protein subunits that are members of the family of proteins generally referred to as neurotransmitter-gated ion channel proteins. Included in this family are the serotonin 5-HT3 receptor, the gamma-aminobutyric-acid (GABA) receptor subunits, including gamma-1, rho-3, and beta-like, and the acetylcholine receptor protein subunits, including alpha-9 chain, epsilon chain, and beta-2 chain.
The neurotransmitter-gated ion channel superfamily includes 5-HT3, GABAA, glutamate, glycine, and nicotinic acetylcholine receptor families. Within this superfamily, functional receptors are formed by homo- or heteropentamers of subunits having four transmembrane domains and an extracellular ligand-binding domain. The transmembrane domains of these receptors contribute to the formation of an ion pore.
Serotonin, also known as 5-hydroxytryptamine or 5-HT, is a biogenic amine that functions as a neurotransmitter, a mitogen and a hormone (Conley (1995) The Ion Channels FactsBook Vol. 1. Extracellular Ligand-Gated Channels, Academic Press, London and San Diego. pp. 426). Serotonin activates a large number of receptors, most of which are coupled to activation of G-proteins. However, 5-HT3 receptors are structurally distinct and belong to the neurotransmitter-gated ion channel superfamily. 5-HT3 receptors are expressed both pre- and post-synaptically on central and peripheral neurons. Post-synaptic 5-HT3 receptors achieve their effects by inducing excitatory potentials in the post-synaptic neuron, whereas pre-synaptic 5-HT3 receptors modulate the release of other neurotransmitters from the pre-synaptic neuron (Conley, 1995). 5-HT3 receptors have important roles in pain reception, cognition, cranial motor neuron activity, sensory processing and modulation of affect (Conley, 1995). Thus, ligands or drugs that modulate 5-HT3 receptors may be useful in treating pain, neuropathies, migraine, cognitive disorders, learning and memory deficits, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, emesis, cranial neuropathies, sensory deficits, anxiety, depression, schizophrenia, and other affective disorders.
Nicotinic acetylcholine receptors (AChR) are distinguished from other acetylcholine receptors by their affinity for nicotine and their structure—homo- or hetero-pentamers like all members of the neurotransmitter-gated ion channel superfamily. Nicotinic AChRs are found at the neuromuscular junction on skeletal muscle and on peripheral and central neurons. These receptors form nonselective cation channels and therefore induce excitatory currents when activated. Nicotinic AChRs are receptors for anesthetics, sedatives, and hallucinogens (Conley, 1995), and certain ligands have shown improvements in learning and memory in animals (Levin et al., Behavioral Pharmacology, 1999, 10:675–780). Thus, ligands or drugs that modulate nicotinic AChRs could be useful for anesthesia, sedation, improving learning and memory, improving cognition, schizophrenia, anxiety, depression, attention deficit hyperactivity disorder, and addiction or smoking cessation. Expression of AChR subunits is regulated during development enabling the design of ligands or drugs specifically targeted for particular developmental stages or diseases.
The neurotransmitter γ-aminobutyric acid (GABA) activates a family of neurotransmitter-gated ion channels (GABAA) and a family of G protein-coupled receptors (GABAB) (Conley, 1995). GABAA receptors form chloride channels that induce inhibitory or hyperpolarizing currents when stimulated by GABA or GABAA receptor agonists (Conley, 1995). GABAA receptors are modulated by benzodiazepines, barbiturates, picrotoxin, and bicucuilline (Conley, 1995). Thus, ligands or drugs that modulate GABAA receptors could be useful in sedation, anxiety, epilepsy, seizures, alcohol addiction or withdrawal, panic disorders, pre-menstrual syndrome, migraine, and other diseases characterized by hyper-excitability of central or peripheral neurons. The pharmacology of GABAA receptors is affected by changing the subunit composition of the receptor. GABA receptor rho subunits are relatively specifically expressed in the retina (Cutting et al., 1991, Proc. Natl. Acad. Sci. USA, 88:2673–7), and the pharmacology of rho receptor homomultimers resembles that of so-called GABAC receptors (Shimada et al., 1992, Mol. Pharmacol. 41:683–7). Therefore, GABA receptors consisting of rho subunits may be useful targets for discovering ligands or drugs to treat visual defects, macular degeneration, glaucoma, and other retinal disorders.
Potassium channels are proteins that form a pore allowing potassium ions to pass into or out of a cell. Potassium channels are comprised of an alpha- (or pore-forming) subunit, and are often associated with a beta- subunit. Three types of potassium ion pore-forming alpha-subunits have been described, exemplified by the Shaker channel (Jan, L Y and Jan, Y N. Voltage-gated and inwardly-rectifying potassium channels. J. Physiol. London 1997; 505:267–282), the inward-rectifier (ibid), and the two-pore (Fink M., Duprat, F., Lesage, F., Reyes, R., Romey, G., Heurteaux, C. and Lazdunski, M. Cloning, functional expression and brain localization of a novel outward rectifier K channel, EMBO J. 1996; 15:6854) channels. There are at least several members in each of these pore-forming families. These pores are comprised of a characteristic number of transmembrane-spanning domains; six transmembrane-spanning domains (Shaker), four transmembrane-spanning domains (two-pore) or two transmembrane-spanning domains (inward rectifier). Transmembrane-spanning domains are regions of the protein that traverse the plasma membrane of the cell. Hence, potassium channels with a Shaker-type alpha subunit are sometimes referred to as 6Tm-1P (for 6 transmembrane-spanning domains-i pore), inward-rectifier channels as 2Tm-1P and two-pore channels as 4Tm-2P.
The 4Tm-2P family of potassium channels was initially discovered in the nematode C. elegans (Salkoff, L. and Jegla, T. 1995, Neuron, 15: 489), but have also been found in yeast, Drosophila melanogaster, bacteria, plants and mammalian cells (Lesage F and Lazdunski M. (1999). “Potassium Ion Channels, Molecular Structure, Function, and Diseases” in Current Topics in Membranes 46; 199–222 ed. Kurachi, Y., Jan, LY., and Lazdunski, M.). In addition to the different biophysical characteristics described above the 4Tm-2P family of potassium channels have different physiological characteristics as well. For example they are regulated by H+ ions, extracellular K+ and Na+ ions, and also by protein kinase c and protein kinase a activators. 4Tm-2P potassium channels are time and voltage-independent, and thus remain open at all membrane potentials. Because of this, these potassium channels are postulated to be responsible for the background potassium ion currents that are thought to set the resting membrane potential (Lesage et al., (1999), “Potassium Ion Channels, Molecular Structure, Function, and Diseases” in Current Topics in Membranes 46; 199–222 ed. Kurachi, Y., Jan, L Y., and Lazdunski, M.).
Potential uses for the channels described herein include the discovery of agents that modify the activity of the channels. Two previously described members of this family (TASK and TREK-1) are activated by volatile general anesthetics such as chloroform halothane and isoflurane (Patel et al., Nature Neuroscience, 1999, 2:422–426), implicating these channels as a site of activity for these anesthetics. In addition, compounds that modify the activity of these channels may also be useful for the control of neuromotor diseases including epilepsy and neurodegenerative diseases including Parkinson's and Alzheimer's. Also compounds that modulate the activity of these channels may treat diseases including but not limited to cardiovascular arrhythmias, stroke, and endocrine and muscular disorders.
Therefore, ion channels may be useful targets for discovering ligands or drugs to treat many diverse disorders and defects, including schizophrenia, depression, anxiety, attention deficit hyperactivity disorder, migraine, stroke, ischemia, and neurodegenerative disease such as Alzheimer's disease, Parkinson's disease, glaucoma and macular degeneration. In addition compounds which modulate ion channels can be used for the treatment of cardiovascular diseases including ischemia, congestive heart failure, arrhythmia, high blood pressure and restenosis. Also, compounds which modulate ion channels can be used to treat diseases and disorders including inflammatory bowel disease, irritable bowel syndrome, diverticulitis, polyps, and the like.